This invention relates to polyolefins, and more particularly to polyolefin compositions having improved paintability. Still more particularly, the polyolefin compositions of this invention comprise a highly isotactic polypropylene having an olefin rubber component dispersed therein. Injection molded articles comprising the invented compositions are characterized by good paint adhesion, a layered morphology in the shear zone nearest the surface, and an excellent balance of mechanical properties including stiffness.
Polypropylene has found wide acceptance for use in a great variety of applications for its ease of molding, good heat resistance and mechanical properties. Resin formulations based on polypropylene may be tailored to meet the demands imposed by a variety of structural and decorative uses in the production of molded parts for appliances, household goods and autos. Impact modified polypropylene and elastomeric ethylene-propylene copolymers have found application in automotive applications including interior trim as well as in exterior parts such as bumper fascia, grill components, rocker panels and the like. Polypropylene resins have the thermal and chemical resistance to withstand exposure to the wide variety of environments encountered in automotive uses, and are easily molded at a cost far below that of metal stamping to provide parts that will not rust or corrode and are impact resistant, even at low temperature.
However, polyolefins are generally known to have poor paintability characteristics. Adhesion of paints, coatings and adhesives to the surfaces of parts molded from these nonpolar, crystalline or semicrystalline resins is generally considered to be very poor, and further treatment or modification is needed for parts to be acceptable for most uses requiring paintability. Considerable research has been directed to developing chemical and physical methods for overcoming this deficiency. Etching, chemical oxidation, treating with polar chemical primers, and the like have been employed for modifying the surface properties. Activation of molded part surfaces through treatment with flame, arc or plasma spray techniques has also been employed for these purposes, and additives to improve paint adhesion to polyolefins have been disclosed in the art.
For the purposes of this invention, the term paintability will be understood to mean the adhesion or lack of adhesion of paints to the surface of a molded plastic article. There are a variety of test methods, both non-standardized and standardized, for determining paintability of plastics. The particular test employed by an industry or a segment of an industry will be selected to measure paint adhesion under practical conditions related to the intended use. In the automotive industry, for example, tests are developed by individual manufacturers to determine adhesion under a variety of field conditions including water immersion, erosion and abrasion by gravel, and stone impact. In these discussions, paintability comparisons will be understood to be based on the same test methods and for panels coated with the same paint or coating materials.
The relationship between paintability and morphology in molded articles comprising blends of polypropylene compounded with impact modifying elastomers has long been the subject of intensive investigation. Generally, rubber-modified polypropylenes are heterogeneous blends and may be further described as comprising a continuous polypropylene matrix phase having a discontinuous rubber phase in the form of particles dispersed therein. When these heterogeneous blends are injection molded shear forces are developed within the molten resin, the stress levels within the molten resin being dependent upon flow rate and melt viscosity, and varying with proximity to the part surface. A cross section of the molded part, when examined microscopically, may be seen to have a shear zone nearest-the surface where the dispersed rubber particles are oriented along the lines of flow and distorted by the flow to have a high aspect ratio. Rubber particles in the bulk or core of the molded part, that portion furthest from the surface, are subjected to lower shear forces and are thus more randomly distributed and tend to be less elongated. Cooling rates within the molded part, slower in the core zone, tend to allow elongated particles in the bulk material to relax and become more spherical in shape. A morphology gradient may be seen in the distribution of the rubber particles in the shear zone, with the oriented rubber phase particles diminishing and becoming more random with distance from the part surface. A distinct layered morphology may be developed along the flow lines. The morphology of the shear zone is believed to have significant effect on surface characteristics, including paintability.
In discussing the relationships between part morphology and paintability, the morphology being referenced and discussed will be understood to be the morphology of the shear zone for an injection molded article, including the surface layer and extending into the bulk or core. The concepts will be well understood by those skilled in the art, and are disclosed and fully described in the art including in the "Polypropylene Handbook" Hanser Publications, NY, 1996, and in the many papers cited therein.
At the surface of the molded article there may be found a layer consisting substantially of crystalline polypropylene (PP). This surface layer or skin, also found for moldings from homopolypropylene, is important to part appearance and surface hardness. The thickness and crystallinity of the PP surface layer that forms depends in part upon molding conditions including mold temperatures and cooling rates, and upon annealing. Where the layer is thick and highly crystalline the part may delaminate when later subjected to impact or to other stress, particularly in the shear direction.
The skin layer may also affect paintability of rubber modified PP resins. The presence of a thick and highly crystalline PP surface layer is disclosed in the art to hinder access of the paint components to the rubber component that lies directly beneath the surface, thereby preventing good adhesion.
Rubber-modified polyolefins with higher levels of rubber modifier are also widely known. These resins, frequently termed thermoplastic polyolefin (TPO) resins, will for the purposes of this invention be generally considered to be rubber modified polyolefins containing greater than about 20 wt % of the rubber component. The morphology of molded thermoplastic polyolefins has been summarized and is well described in the art, particularly in Organic Coatings 27 (1996) pp. 241-254; the teachings of this reference are incorporated herein in their entirety by reference.
Morphology is believed by those skilled in the art to similarly affect the paintability of TPO resins. Articles molded from compositions comprising the higher levels of rubber found in TPO resins tend to form more highly layered structures, with crystalline and immiscible or poorly miscible components. These layered structures are more likely to undergo cohesive failure within the matrix, and the interface between the crystalline PP forming the surface layer and the rubber modified layer immediately below the surface may be particularly susceptible to immiscibility. Thick, highly crystalline PP surface layers will thus be particularly subject to such failure, with shearing impact resulting in delamination of the surface layer. Resistance to stone impact, a highly important property for automotive exterior parts, is disclosed in the art to be highly dependent on resistance of the surface layer to cohesive failure.
Good dispersion of the rubber component in the homopolymer matrix is important to mechanical properties as well as to melt rheology and thereby to the morphology of molded articles. Compounding processes are widely used to disperse the rubber component into the PP matrix, and methods for achieving adequate dispersion are well developed. The size of the rubber particles will be controlled by molecular weight of the rubber component which in turn is a direct function of the polymer viscosity. The effect of viscosity ratio--the ratio of rubber viscosity to matrix viscosity--on rubber dispersion and morphology is well documented in the art. Increasing the viscosity ratio has been disclosed to increase particle size for blends, while a lowered ratio tends to result in an increase in the aspect ratio for the rubber particles and in a greater degree of rubber phase orientation near the surface. Proper selection of viscosity ratio has also been disclosed in the art to reduce or prevent agglomeration of rubber particles during processing, particularly for reactor-generated, rubber modified polypropylenes.
More recently, in-reactor methods have been developed for producing high rubber content thermoplastic polyolefins akin to the TPO blends. The rubber component of thermoplastic polyolefin resins produced by these processes, termed RTPO resins, is finely and uniformly dispersed in the PP matrix and without need for subjecting the resins to a compounding operation. In-reactor polymerization processes also afford good control of the rubber component composition, and of the molecular weight for the initial homopolymer and the final resin product. The processes permit the resin producer to tailor the overall balance of stiffness, hardness, impact resistance, and tensile strength properties for RTPO resins to meet the specific requirements of a wide range of applications.
Good surface properties including paintability are a further requirement of resins intended for use in the production of appearance parts and in trim and decorative uses, particularly in automotive applications. TPO resins having the desired paintability are difficult to produce reliably, and none are presently available with the desired combination of stiffness, paintability and durability needed for automotive uses. RTPO resins having acceptable paintability over a wider range of stiffness and rigidity, together with good durability in use of the painted articles are needed and such materials would readily find wide acceptance in the molding arts. A method for reliably producing such resins would provide a widened range of materials particularly suited for use in exterior and interior trim and in appearance parts where painting is contemplated and would be an important advance in the art.